Radio frequency pulse generating apparatus using an exploding wire



Oct. 26, 1965 R. F. WUERKER 3,214,707

RADIO FREQLDNCY PULSE GENERATING APPARATUS USING AN EXPLODING WIRE Filed Jan. 11, 1962 2 Sheets-Sheet 1 .-0 I Q 5! J16 RALPH /-T MMERKER '\\\\\\\\\\\Ro INVENTOR.

/ 2 BY 9! G AGENTS Oct. 26, 1965 R. F. WUERKER 3,214,707 RADIO FREQUENCY PULSE GENERATING APPARATUS USING AN EXPLODING WIRE Filed Jan. 11, 1962 2 Sheets-Sheet 2 I I 5'5" 2 I I I I I I Hi I Q Z I 1 I I I i I TRAN$ITION I LLI I! II I J M-" I /a) I o 'I'JI {I I TIME I METALLIC I CONDUCTIOII EE (\I II I I I GASEOUS METALLIC Ix I' IJERIQD I CONDUCTION 0 II I 5 I: f: I 713 II/"v 1 VCSCLP'IILC) U-I Z 46 I/f 48 I 3 U 3 I O I! I I I (b) I I I F 271- II LP C RALPH A Wl/f/Q/(EQ INVENTOR.

BY M a A GENTS United States Patent 3,214,707 RADIO FREQUENCY PULSE GENERATING AP- PARATUS USING AN EXPLODING WIRE Ralph F. Wuerker, Palos Verdes Estates, Califi, asslgnor, by mesne assignments, to TRW Inc., a corporation of Ohio Filed Jan. 11, 1962, Ser. No. 165,686

8 Claims. ((31. 331-428) This invention relates to radio frequency power generation, and more particularly to a method of and means for generating radio frequency pulses containing megawatts of power and having a duration of microseconds.

In making certain studies in thermonuclear and plasma physics, it has been found necessary to provide a source of radio frequency pulses of short duration and high power. Conventional means for generating such pulses, such as large thermionic tubes, are expensive to build. Less elaborate generating means, such as a simple capacitor, discharged through a triggered switch (1.e., ignitron, thyratron, spark gap) suffer from inherently low Q due to resistive losses in the switch.

Accordingly, an object of this invention is to provide a simple, economical means for generating radio frequency pulses containing megawatts of power and having a duration of microseconds.

A further object of this invention is to generate high power radio frequency energy which oscillates with a relatively high Q for at least a period of several microseconds.

According to the invention, a parallel resonant circuit, or resonator, is connected in series with an explodable wire, a switching device, and an energy storage device. The energy storage device is initially charged to a high voltage and the switching device is maintained open. To generate the high power frequency pulses, the switching device is closed, whereupon the energy storage device starts to discharge into the resonator through the explodable .wire. For the brief interval during which the explodable wire is intact and conducting, the initial surge of discharge current inductively charges the resonator. The flow of current heats the wire, causing it to abruptly transform from a solid conductor to a nonconducting metallic vapor for a brief interval known in exploding wire technology as the dwell period. This action isolates the resonator from the charging circuit, and the resonator goes into oscillation, thereby generating the desired high power radio frequency train, with a decay time determined only by the losses in the capacitive and inductive portions thereof. The resonator continues to serve as a .source of radio frequency power until the metallic vapor, on expansion, breaks down and again conducts current between the resonator and charging circuit.

-In'the drawing:

FIG. 1 is a schematic circuit illustrating one embodiment of aradio frequency generator according to the invention,

FIG. 2 shows graphs illustrating the resistance versus time characteristic of an exploding wire and the resulting current through the inductive portion of the resonator, FIG. 3 is a schematic circuit illustrating a modified form of a frequency generator according to the invention, and

FIG. 4 is a sectional view showing an improved construction of an explodable wire.

FIG. 1 is a schematic circuit showing one embodiment of a high power radio frequency generator according to the invention. A resonator comprises a parallel tuned circuit of an inductor 12 and a capacitor 14. The resonator 10 is connected in a charging circuit which includes an energy storage device 16, such as a high voltage capacitor or a pulse forming network, a switching device 18, such as a thyratron, ignitron, or a spark gap, and an explodable wire 20 bridging the gap between two spaced electrodes 22 and 24. The two electrodes 22 and 24 and the explodable wire 20 constitute an exploding wire switch. As used herein, the term wire as applied to the exploding wire switch is intended to include wire of cylindrical shape, ribbons, tubes, or any geometrical form in which a conductor can be fabricated.

The energy generated in the resonator 10 may be applied to a load 26 through an inductive element 28 coupled to the inductor 12. Alternatively, either the inductor 12 or the capacitor 14 may constitute the load.

The energy storage device 16 is charged to a voltage of several tens of kilovolts from a direct current source 30 through a resistor 31. After the energy storage device 16 is charged, the source 30 can be disconnected by opening a switch 32.

The switching device 18, illustrated as a thyratron, has its anode 34 connected to the energy storage device 16 and its cathode 36 connected to an electrode 22 of the exploding wire switch. The grid 40 is normally maintained at the cathode 36 potential by connection through a grid bias resistor 42. The normally nonconducting switching device 18 is triggered into conduction by a positive voltage pulse 44 applied to the grid 40.

The present invention, as stated earlier, utilizes the resistive characteristics of an exploding wire to produce a switch for abruptly disconnecting the resonator 10 from the energy storage device 16. The phenomena of exploding wires is treated extensively in the book EX- ploding Wires, by W. G. Chase and H. K. Moore, Plenum Press, New York, New York, 1959.

In operation, the energy storage device 16 is initially charged to a high voltage of magnitude depending upon the amount of power it is desired to generate. Upon triggering of the switching device 18, the energy storage device 16 is conductively connected in series with the resonator 10 through the explodable wire 20. Current is established in the inductor 12 during the period that the current flows through the explodable wire 20.

The latter is normally conducting so that the resonator 10 receives current from the energy storage device 16.

Referring to graph (a) of FIG. 2, which shows the variation in resistance of an exploding wire as a function of time, the interval between t and t corresponds to the initial period during which the wire 20 experiences metallic conduction, or is normally conducting. This initial period lasts typically for a few tenths of a microsecond.

In graph (b) of FIG. 2, curve 46 shows the current through the inductor 12 during the period between t and t The current through the inductor rises slowly at a frequency (1) determined by the capacitance C, of the energy storage device 16, the inductance L of the resonator, and all other inductances L in the circuit.

As the charging current builds up, the explodable wire 20 is heated to the point where it abruptly transforms from a conductor to a highly dense nonconducting superheated aerosol. time of transition 1 the resistance jumps by many orders of magnitude and remains at a high level for a second interval known as the dwell period. In the graphs of FIG. 2, the interval between 1 and t corresponds to the dwell period.

Since the resistance of the exploding wire gap between the electrodes 22 and 24 is so high during the dwell period, the circuit between the energy storage device 16 and the resonator 10 is effectively broken. Isolated from the charging circuit, the resonator 10 begins to oscillate at a frequency (f,) determined by its circuit constants At this point, corresponding to the.

L, and C as shown in curve 48 of graph (b). The de cay of the oscillations is determined only by the normal circuit losses in the inductor 12 and capacitor 14 of the resonator 10.

The pure oscillations persist until the density of the metallic vapor in the exploding wire switch reduces to the point where gaseous reconduction can be established in the gap between electrodes 22 and 24. Voltage breakdown of the gap results from the radial expansion of the heated metal vapor cloud. As the column of metal-vapor gas expands, the density decreases with the result that the voltage also decreases, in accordance with the Paschen breakdown curves for gas. Typical Paschen curves are shown in the American Institute of Physics Handbook, 1957 McGraw-Hill Book Co., Inc., N.Y., Sec. 5, p. 179. When the density reaches a critical value determined by gap spacing, vapor temperature and gap voltage reconduction is abruptly established and the resonator is once again electrically connected to the energy storage device 16. The time at which conduction reoccurs is known as the restrike time and correponds to time t in the graphs. At this time t current is re-established between the energy storage device 16 and the resonator 10. The current, as depicted in curve 47, oscillates at the original low frequency (7) (dashed line curve 46a), upon which is superimposed the trapped radio frequency current in the resonator 10.

After restrike, the purity of the radio frequency wave form 48 is severely distorted by the flow of current from the energy storage device 16 through the resonator 10. This undesirable result can be avoided by dissipating the remaining stored energy in the energy storage device 16 before restrike occurs. Toward this end, FIG. 3 shows the addition of a shunting circuit across the energy storage device 16. The shunting circuit may comprise a switch tube 50, such as thyratron or ignition, or a spark gap, and a series resistor 52 of a resistance value equal to the characteristic impedance of the energy storage device 16. The switch 50 has its anode 54 connected to the high voltage terminal of the energy device 16 and its cathode 56 connected through the series resistor 52 to the low voltage terminal of the energy storage device 16. A bias resistor 62 is connected between the grid 58 and the cathode 56.

The switch tube 50 is automatically made conducting by application to its grid 58 of a pulse 60 derived from an external source.

In accordance with one operative embodiment, a high voltage capactor 16 of .25 microfarad charged to 30 kilovolts was connected in series with an explodable wire and a resonator 10. The electrodes 22 and 24 consisted of two solid brass spheres 1 inch in diameter. The exploding wire gap between electrodes 22 and 24 was 3 inches wide and included an intermediate opaque, dielectric barrier in the form of a Micarta sheet, 2 inches high and inch thick. A length of No. 36 copper wire was strung over the barrier sheet and held taut between the two electrodes 22 and 24 to serve as the exploding wire 20. The barrier served to lengthen the dwell period by about 20%. The resonator 10 included a 0.155 microhenry inductor 12 and a .029 microfarad capacitor 14, the resonant frequency of this circuit being 2.74 megacycles per second. Measurements showed that after the explosion of the wire 20, the resonator 10 oscillated for a period of 6 microseconds with a Q, or a figure of merit, of 88. The peak current attained in the radio frequency pulses was 7.2 kiloamperes, representing 52 megavolt-amperes of circulating apparent power in the resonator 10.

In the foregoing operative embodiment there was a 2.5% transfer of energy from the low freqency charging circuit to the resonator 10. The amount of energy transfer is strongly dependent on the ratio of the period of metallic conduction (t to t to the period of switchoff time at (FIG. 2),. The switch-off time at is the time required for the current flowing through the wire 20 to decrease to a very low value. Efficient operation requires that 5t 21r /L,.C,./ 4, or that the switch-off time be less than A of the period of oscillation of the resonator. The larger the above ratio, the higher the energy transfer. It has been found that, in general, for a given wire diameter and a given charging capacitance, the switch-off time decreases with both a decrease in the total length of the wire 24 and an increase in the voltage on the charging capacitor 16.

The dwell time can be increased by physically packing the wire 20 in an electrical insulating material, such as sand or plastic, for example. The purpose of the packing is to keep the metallic vapor density high, and thus nonconducting, for as long a time as possible, in order to isolate the resonator 10 from the charging circuit. By this means it is possible to prolong the dwell time, and thus the period of oscillation, indefinitely.

FIG. 4 shows how the wire 20 can be packed to increase the dwell time. The explodabie wire 20 is encased within a metallic tube 64, made of steel, for example, and the space between the tube 64 and the wire 20 is filled with electrical insulating material 66. The ends of the tube 64 are provided with insulating flanges 68, such as Teflon, or plastic, which provide protection against voltage flash back between the two electrodes 22 and 24.

It is now apparent that the invention provides a relatively simple and economical means for generating high power radio frequency pulses of short duration and slow decay.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A radio frequency power generator, comprising: a series circuit including,

a high voltage capacitor adapted to receive an initial charge of at least several kilovolts,

an initially open switching device,

an explodable wire connected across a pair of electrodes, and

a parallel resonant circuit;

and means for initially charging said capacitor to several kilovolts;

said explodable wire providing a conductive path for current for charging said resonant circuit from said high voltage capacitor for a brief interval following the closing of said switching device,

said current causing said explodable wire to abruptly transform to a nonconducting metallic vapor for a second interval to provide a high resistance path for interrupting the flow of charging current, whereby said resonant circuit produces self sustained oscillations during said second interval.

2. A radio frequency power generator comprising:

a high voltage capacitor,

voltage source means in series with said capacitor for charging said capacitor to a potential on the order of 30 kilovolts,

a pair of spaced electrodes bridged by an explodable wire,

a radio frequency parallel resonat circuit, and

a switching means operative to connect said parallel resonant circuit, said wire, and said capacitor in a series circuit to cause current to be conducted between said capacitor and said parallel resonant circuit during an initial time period when said wire has relatively low resistance,

said wire being transformed to a dense metallic vapor by said current to raise the resistance across said electrodes to a relatively high value during a second time period, thereby to terminate current flow in said series circuit and to generate desired radio frequency oscillations in said parallel resonant circuit during said second time period.

3. A high frequency power generator, comprising:

an energy storage device,

voltage source means in series with said energy storage device for charging the same to a potential of at least several kilovolts,

an explodable wire,

a parallel resonant circuit,

means including a switching device connecting said energy storage device, said explodable wire and said resonant circuit in series, and

a load coupled to said resonant circuit for receiving energy therefrom,

said explodable wire serving temporarily as a conductor for conducting current from said energy storage device to said resonant circuit, thereby to store energy in said resonant circuit,

said current causing an abrupt transition in said explodable wire from a conducting to a nonconducting state to isolate said resonant circuit from said storage device, whereupon said resonant circuit oscillates at a frequency determined by its circuit parameters.

4. The invention according to claim 3, wherein said load is coupled as a separate element to said resonant circuit.

5. The invention according to claim 3, wherein said load is included as a reactive element of said resonant circuit.

6. A high frequency power generator, comprising:

a charge storage device;

voltage source means connected in series with said charge storage device for charging the same to a potential of at least several kilovolts;

an explodable wire;

a tank circuit;

switching means energizable to connect said storage device, said wire, and said tank circuit in series; and

a shunting circuit in parallel with said storage devce;

said explodable wire being initially in a conducting state for a brief interval following energizing of said switching means, so as to conduct current from said storage device to said tank circuit;

said current causing said wire to transform abrutly to a nonconducting vapor state to isolate said tank circuit from said storage device and to cause said tank circuit to produce a wave train for the period said wire is nonconducting;

said shunting circuit including a dissipative load matched to the characteristic impedance of said storage device, and

a normally open switching device connected in series with said dissipative load, said switching device being arranged to close during the period said wire is nonconducting to connect said storage device in circuit with said dissipative load. 7. The invention according to claim 6, wherein said switching means comprises a thyratron having an anode connected to the positive side and a cathode connected through said wire and said tank circuit to the negative side of said charge storage device, and an impedance connected between the grid and cathode of said thyratron.

8. A radio frequency power generator, comprising: a charge storage device; voltage source means connected in series with said charge storage device for charging the same to a potential of at least several kilovolts; an exploding wire switch; a parallel resonant circuit; and a switching means energizable to connect said storage device, said exploding wire switch and said resonant circuit in series; said exploding wire switch includng a pair of spaced electrodes, a vaporizable metal wire connected between said electrodes, a tubular metal sheath surrounding said wire, and a packing of electrical insulation material filling the space between said wire and said sheath.

References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCES Generation of High Power Radio-Frequency Pulses by means of an Exploding Wire Technique, Jones et al.

The Review of Scientific Instruments, Vol. 32, No. 8

August 1961, pages 962-963.

Radar Electronic Fundamentals War Dept, Dec. 30,

5 1943, pages 187-191.

ROY LAKE, Primary Examiner.

JOHN KOMINSKI, BENNETT G. MILLER, Examiners. 

2. A RADIO FREQUENCY POWER GENERATOR COMPRISING: A HIGH VOLTAGE CAPACITOR, VOLTAGE SOURCE MEANS IN SERIES WITH SAID CAPACITOR FOR CHARGING SAID CAPACITOR TO A POTENTIAL ON THE ORDER OF 30 KILOVOLTS, A PAIR OF SPACED ELECTRODES BRIDGED BY AN EXPLODABLE WIRE, A RADIO FREQUENCY PARALLEL RESONAT CIRCUIT, AND A SWITCHING MEANS OPERATIVE TO CONNECT SAID PARALLEL RESONANT CIRCUIT, SAID WIRE, AND SAID CAPACITOR IN A SERIES CIRCUIT TO CAUSE CURRENT TO BE CONDUCTED BETWEEN SAID CAPACITOR AND SAID PARALLEL RESONANT CIRCUIT DURING AN INITIAL TIME PERIOD WHEN SAID WIRE HAS RELATIVELY LOW RESISTANCE, SAID WIRE BEING TRANSFORMED TO A DENSE METALLIC VAPOR BY SAID CURRENT TO RAISE THE RESISTANCE ACROSS SAID ELECTRODES TO A RELATIVELY HIGH VALUE DURING A SECOND TIME PERIOD, THEREBY TO TERMINATE CURRENT FLOW IN SAID SERIES CIRCUIT AND TO GENERATE DESIRED RADIO FREQUENCY OSCILLATIONS IN SAID PARALLEL RESONANT CIRCUIT DURING SAID SECOND TIME PERIOD. 